U.S. Pat. No. 4,768,446, issued Sept. 6, 1988 to the inventors of this invention and assigned to the assignee of this invention, describes an external combustion system for a stationary gas turbine engine. The described combustion system is particularly suitable for burning pulverized coal and includes a rich zone combustor where the coal is burned at an equivalence ratio above 1 and at a temperature above the slagging temperature of the coal. The combustion products of the rich zone combustor, including combustible gas, molten slag, and ash, pass through a coolant screen which quenches the combustion products. The quenched slag and ash are separated from the combustible gas in two stages of inertial separation. The combustible gas is then introduced into a lean zone combustor with additional combustion air. The combustible gas ignites spontaneously and burns at an equivalence ratio less than 1 to produce a substantially ash-free and slag-free stream of hot gas motive fluid suitable for introduction into the turbine stages of the gas turbine engine. A combustion system according to this invention employs the combustion system of the aforesaid U.S. Pat. No. 4,768,446 without inertial separation stages to achieve low levels of oxides of nitrogen and carbon monoxide with non-slag and non-ash forming fuels such as natural gas or liquid hydrocarbons.